This application is based on applications Nos. 98-18312, 98-37047, 98-37048 and 99-16441 filed in the Korean Industrial Property Office on May 21, 1998, Sep. 8, 1998, Sep. 8, 1998 and May 8, 1999, respectively the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a negative active material for a lithium secondary battery and a lithium secondary battery by using the same and, more particularly, to a negative active material which can improve charge density of the electrode and has good high-rate charge-discharge characteristics and cycle life characteristic.
(b) Description of the Related Art
As a negative active material in a lithium secondary battery, particularly, a lithium ion battery or a lithium ion polymer battery, carbonaceous materials are generally used. There are two basic types of the carbonaceous material: crystalline graphite and amorphous carbon. Crystalline graphite includes nature graphite and artificial graphite obtained by sintering pitch at 2000xc2x0 C. Amorphous carbon has a low graphitization degree or diffraction line in X-ray diffraction. Amorphous carbon includes a soft carbon obtained by coal pitch or oil pitch and a hard carbon obtained by a polymer resin such as phenol resin.
Crystalline graphite offers the advantages of exceptional voltage flatness and high charge/discharge efficiency, but a charge/discharge decreases as a result of side reactions if many of the edges of the hexagonal crystalline graphite are exposed to electrolyte. With regard to amorphous carbon, although this material displays a high discharge capacity, it has a high irreversible capacity, low charge/discharge efficiency, and does not display a high level of voltage flatness.
Therefore, crystalline graphite is generally used for the negative active material in the lithium secondary battery. For using crystalline graphite such as nature graphite or artificial graphite in the battery, pulverizing and sieving steps must be performed. In the steps, non-uniform or disk-shaped carbonaceous active material is inevitably produced due to the high crystallinity of crystalline carbon. When produced non-uniform or disk-shaped of the active material is used in an electrode plate, it is to produce a negative electrode having a low tap density. Furthermore, when the active material is coated on the electrode plate and pressed, a basal plane of the active material is only exposed to an electrolyte by orienting non-uniform active material particles. The basal plane has lithium ion absorption and desorption problems, thereby deteriorating high-rate charge/discharge characteristics. Furthermore, the expansion and shrinkage rates of volume the produced electrode increases, lowering cycle life characteristic. Furthermore, graphene sheet is developed in edge of crystalline graphite and the side reaction of graphite and an electrolyte is activated so that it is difficult to use crystalline graphite for a lithium secondary battery requiring high initial efficiency (Journal of Electrochemical Society 137 (1990) 2009). More particularly, if an electrolyte including propylene carbonate is used in a lithium secondary battery including crystalline graphite, the crystalline graphite layer is separated from the electrode because of the co-intercalation of the electrolyte. Due to the reason, lithium ion is difficult to move back and forth of the electrode, decreasing initial efficiency of the active material and capacity of the battery.
There have been attempts at producing carbonaceous material using both crystalline graphite and amorphous carbon together to obtain the advantages of both these materials.
U.S. Pat. No. 5,344,726 discloses a carbon anode for secondary battery prepared by coating amorphous carbon on the surface of high crystalline graphite. The method can controls decomposition of an electrolyte and increases initial charge-discharge efficiency. However, because the carbon anode of the method has a very thin amorphous carbon layer, non-uniform or disk-shaped form of the crystalline graphite do not changed into a spherical form which enables to increase a packing density. Furthermore, in pressing the electrode, the high crystalline graphite core is still totally oriented, lowering high-rate charge-discharge and cycle life characteristics.
U.S. Pat. No. 5,401,598 discloses an electrode including a carbon active material of multi-phasic structure. The multi-phasic structure carbon active material is prepared by adding graphite powder to a mixed solution of pitch and toluene and heating the mixture to obtain a carbonaceous material including graphite core and a pitch surface. The carbonaceous material is formed and carbonized to prepare the carbon active material having multi-phasic structure. The active material has excess of amorphous carbon, at least 35 volume percent. Due to the excess of amorphous carbon, voltage flatness is poor. Furthermore, because the method includes a pulverizing step after carbonizing step, a surface of amorphous carbon may be exposed to an electrolyte. Subsequently, the effect of coating amorphous carbon on crystalline graphite is not shown and initial charge-discharge efficiency decreases as a result of side reactions of graphite and the electrolyte.
Furthermore, a method of adding a conductivity agent to an active material, a method for manufacturing multi-electrode plate by using a metal which can form a metal thin layer with high conductivity and method of mixing at least two type of active materials are attempt to increase charge density, high-rate charge-discharge and cycle life characteristics. However, the effect is not satisfactory.
It is an object of the present invention to provide a negative active material for a lithium secondary battery which can improve packing density without deteriorating electrochemical characteristics such as high-rate charge-discharge characteristics.
It is another object to provide the negative active material having good voltage flatness, initial charge-discharge efficiency, high-rage charge-discharge efficiency and cycle life characteristics.
It is another object to provide a lithium secondary battery using the negative active material.
It is another object to provide the lithium secondary battery having high capacity and efficiency, and good cycle life characteristics without the limitation of the electrolyte type.
These and other objects may be achieved by a negative active material for a lithium secondary battery including a crystalline carbon core and an amorphous carbon shell formed on the core.
In order to achieve these objects and others, the present invention provides a negative active material for a lithium secondary battery comprising secondary particles having a substantially spherical form. The secondary particles is prepared by gathering at least one crystalline carbon primary particle and has an amorphous carbon surface.
The present invention further includes a lithium secondary battery. The lithium secondary battery includes a positive electrode including an positive active material, a negative electrode including an active material having at least two exothermic peaks, a separator interposed between the positive and the negative electrodes, and an electrolyte. The negative active material includes a crystalline carbon and an amorphous carbon. The electrolyte is immersed in the positive and negative electrodes and the separator, and comprises an organic solvent and a lithium salt.
The organic solvent may be at least 51 volume percent of cyclic carbonate and linear carbonate. Alternatively, the organic solvent may be a first solvent of ethylene carbonate, dimethy carbonate and propylene carbonate; a second solvent of ethylene carbonate, dimethyl carbonate, diethyl carbonate and propylene carbonate; and a third solvent of ethylene carbonate, diethyl carbonate and propyl acetate.